1. The Field of the Invention
The present invention relates generally to materials handling and/or classifying equipment, and more particularly to a particulate conveying system utilizing an auger combined with double air injection.
2. The Background Art
Pneumatic conveying systems having auger conveyor sections are known in the art and have been in use for many years for transporting various particulate materials, such as pulverized coal, portland cement, grain, pulverized ore, etc., through pipelines and other conduits. Such systems typically comprise a motor driven auger conveyor rotatably mounted within a barrel, a gravity-fed hopper for supplying particulate material to the auger conveyor through an opening in the barrel, and a mixing chamber situated at the discharge end of the barrel where the material ejected by the auger conveyor is subject to one or more jets of pressurized air or other gas.
The mixture of material and gas thus formed is propelled by the pressurized gas through a conduit connected to the mixing chamber. Conventional pneumatic conveying systems with auger conveyor sections have the drawback that the mixing of the pressurized gas with the particulate material in the mixing chamber produces considerable turbulence in the mixture transported through the conduit. Such turbulence is undesirable in that it increases the frequency of contact of the particulate material with the conduit walls during transport and therefore aggravates frictional and wear losses in the conveying system. Frictional losses require higher rates of energy to transport the material and significantly reduce the distance over which the material can be conveyed. When the material being transported is abrasive or corrosive, contact of such material with the conduit walls increases wearing of the walls and reduces the useful life of the conduit.
Pneumatic conveying systems for particulate materials have also been devised in which the mixing of material with gas takes place in a venturi pipe through which pressurized gas flows. By introducing the particulate material into the relatively high velocity gas flow in the constriction or throat of the venturi pipe, mixing of the material with the gas occurs with little turbulence, and therefore the resulting mixture may be propelled through the conduit with reduced frictional loss and less wear of the conduit walls.
In conventional venturi conveying systems, the particulate material to be mixed with the gas is supplied to the venturi pipe via a chute having a discharge end located near the constriction of the venturi pipe, where a vacuum (negative pressure) created by the high velocity gas flow therein acts to draw the material from the chute into the venturi pipe. Because the material being transported does not always flow freely in the chute, the transport rate in a conventional venturi conveying system is difficult to control and is subject to variation caused by changes in the properties of the material being conveyed, such as density, moisture content, particle size, etc. Moreover, a conventional venturi conveying system may not provide a satisfactory material-to-air conveyance ratio for materials that are not sufficiently free flowing in the chute to allow the negative pressure in the venturi pipe to draw such materials from the chute at an adequate rate.
One attempt to improve the conventional venturi conveying systems is described in U.S. Pat. No. 4,711,607 (issued on Dec. 8, 1987 to Wynosky et al.). Because the transport rate through the auger portion of the system is difficult to control, the patent teaches the use of special high pressure jet nozzles in addition to the venturi piping to prevent blow back and product plugs from occurring. The patent also teaches sensors for determining the air pressure difference between an air plenum chamber surrounding the auger barrel and the conduit at the end of the venturi restriction, so as to regulate the product flow through the auger and into the venturi mixing chamber by means of a variable speed motor driving the auger. The system taught in the Wynosky patent does not solve the problems of conveying the material at an adequate rate, and suffers from sever mechanical difficulties related to product blockage and blow back and an inadequate and inconsistent flow rate.
Thus both the conventional auger conveyor systems, the conventional venturi conveying systems, and the venturi systems having adjustable venturi parts and air jet nozzles are subject to the problem of "blowback", which occurs when there is a blockage in the conduit. The blockage results in excessive pressure in the conduit and produces an undesirable back flow of gas and product in the hopper of the auger conveyor system and in the chute or hopper of the venturi conveying system.
Prior attempts to solve the problem of backflow include the use of a flapper valve or an air lock of the auger barrel or in the chute of the venturi system to prevent back flow of gasses and product when the pressure in the conduit becomes excessively high. Such mechanical devises are subject to binding, clogging and wear from contact with abrasive or corrosive materials and therefore require substantial maintenance.